A. Field of the Invention
This invention relates to telephony communications and related telephone bus protocols.
B. Related Art
A digital telephone may conventionally include at least two types of ports: a telephone line interface for connection with a telephone network, and a data communications interface for communications with a computer. For example, the ROLMphone (R) 244PC includes a telephony port for providing connection to a telephone network switch, such as a private branch exchange (PBX), and two data communications ports for providing connection to a computer.
In conventional telephones, the telephony communications link may be provided using a variety of protocols. For example, in the 244PC the connection to the network switch is accomplished by way of ROLMlink protocol. In many other conventional telephones, the connection to the switch is established by using an industry standard protocol such as ISDN.
A ROLMlink connection carries four types of information: digital data communications control, digital data communications, digital voice and general control. Digital data communications control refers to information concerned solely with controlling digital data connections and status. General control is concerned with control of the telephone as a whole. The information is organized into "fields" (groups of bits associated with one of the four information types). The fields are concatenated into a ROLMlink "frame" which contains one of each of the fields. Transfers on the link from the switch to the phone are said to be "downlink" while transfers from the phone to the switch are said to be "uplink".
Once the information arrives at the telephone, it is distributed to the telephone's various components by way of an internal bus. Conventionally, the telephony link is connected to the internal bus by way of a link interface, which translates the telephony information into the proper format and timing in accordance with the internal bus standard.
The internal bus may be one of a number of types. A conventional standard for such internal busses is General Circuit Interface (GCI). The GCI standard, describes, among other things, the structure of data frames and the timing of certain signals.
In prior art digital telephones, digitally encoded audio information arriving from the telephony link is sent from the link interface to a coder/decoder (CODEC). The CODEC is a digital-to-analog and analog-to-digital converter, specialized for telephone applications. The CODEC converts digital audio information into analog form to drive a speaker and/or handset, and converts the analog information from a microphone or handset to digital information destined for the telephone network switch.
Typically, the CODEC uses a timing pulse, generated by the link interface, to identify the particular time slot designated to hold the incoming or outgoing digital audio information within each data frame. Since the audio information is typically located in a fixed time slot within every frame, the link interface is conventionally set up once, at initialization time, to assert the timing pulse in coincidence with this fixed time slot.